This invention relates to the microfabrication of a circuit path on probes for use in probe-based instruments, and more particularly, to methods to integrate a simple circuit path from a probe handle and over the cantilever and back. It also relates to applications where sensing or initiating a mechanical response in the probe is made via the probe's integrated circuit.
For the sake of convenience, the current description focuses on probes that may be realized for a particular embodiment of probe-based instruments, the atomic force microscope (AFM). Probe-based instruments include such instruments as AFMs, 3D molecular force probe instruments, high-resolution profilometers (including mechanical stylus profilometers), surface modification instruments, chemical or biological sensing probes, and micro-actuated devices. The probes described herein may be realized for such other probe-based instruments.
An AFM is an instrument used to produce images of surface topography (and/or other sample characteristics) based on information obtained from scanning (e.g., rastering) a probe relative to the surface of the sample. Probes consist of a handle by which the probe is fastened to the frame of the AFM, a cantilever extending from the handle and a sharp tip at the end of the cantilever. Topographical and/or other features of the surface are detected by sensing changes in the probe's mechanical response to surface features and using feedback to return the system to a reference state. By scanning the probe relative to the sample, a “map” of the sample topography or other sample characteristics may be obtained.
Changes in the probe's mechanical response are typically detected by an optical lever arrangement whereby a light beam is directed onto the cantilever in the same reference frame as the optical lever. The beam reflected from the cantilever illuminates a position sensitive detector (PSD). As the probe's mechanical response changes, a change in the output from the PSD is induced. These changes in the PSD signal are typically used to trigger a change in the vertical position of the base of the probe relative to the sample (referred to herein as a change in the Z position, where Z is generally orthogonal to the XY plane defined by the sample), in order to maintain a constant pre-set value for one or more of the probe's mechanical responses. It is this feedback that is typically used to generate an AFM image.
AFMs can be operated in a number of different sample characterization modes, including contact mode where the tip of the probe is in constant contact with the sample surface, and AC modes where the tip makes no contact or only intermittent contact with the surface. AC modes are typically achieved by mechanically coupling the probe to a piezo-electric element which shakes the probe at a desired frequency, commonly the resonant frequency of the cantilever. These two modes define two mechanical responses of the probe that can be used in the feedback loop which allow the user to set a probe-based operational parameter for system feedback.
In contact mode the interaction between the probe and the sample surface induces a discernable effect on a probe-based operational parameter, such as the cantilever deflection. In AC mode the effects of interest include the cantilever oscillation amplitude, the phase of the cantilever oscillation relative to the signal driving the oscillation and the frequency of the cantilever oscillation. All of these probe-based operational parameters are detectable by a PSD and the resultant PSD signal is used as a feedback control signal for the Z actuator to maintain the designated probe-based operational parameter constant.
The feedback control signal also provides a measurement of the sample characteristic of interest. For example, when the designated parameter in an AC mode is oscillation amplitude, the feedback signal may be used to maintain the amplitude of cantilever oscillation constant to measure changes in the height of the sample surface or other sample characteristics.
Probes are micro-electrical mechanical systems (MEMS) microfabricated by using semiconductor fabrication techniques as this provides a way to batch produce probes with consistent cantilever and tip geometries necessary for use with AFMs today. These techniques include, but are not limited to: thin film deposition, photolithography with optical masks, Reactive Ion Etching (RIE) with plasma, anisotropic wet etching of silicon, and wafer-to-wafer bonding. Silicon and silicon nitride are the two primary semiconductor materials from which AFM probes are fabricated; silicon probes tend to have higher resonant frequencies and higher force constants than silicon nitride probes.
Relatively little development has gone into the advancement of probe technology as compared to other components of AFMs. Many thin films that have been deposited on standard probes that have increased performance or added some functionality, but the basic AFM probe technology remained. Probes have become a bottleneck in the overall advancement of AFM technology.
The life sciences have become an important application for the AFM. Frequently this requires a fluid environment. Using AC modes in such an environment poses challenges because the piezo-electric element, commonly used as the oscillator, can short out in fluid. To overcome this challenge, the holder of the probe, and therefore the cantilever and the tip, together with the fluid have all been shaken, with the piezo-electric element isolated from the fluid. However, image quality and stability have greatly suffered with this configuration because the cantilever's true resonance is hidden in a myriad of resonances coupled to the cantilever via the fluid. AC mode in fluid would perform better if only the cantilever were oscillating and not the holder of the probe and the fluid.
More generally the current technology suffers from the fact that the piezo-electric element, commonly used as the oscillator, operates mechanically. Since the piezo-electric element is coupled to the AFM its operation causes unwanted vibrations that translate to noise in the images produced by the AFM. As probes get smaller and faster in order to decrease imaging time, this will become an even bigger issue. AC mode would perform better, and with a lower noise floor, if the probe could be actuated without being coupled to a mechanical actuator.
Efforts have long been made to improve upon the optical lever arrangement whereby a light beam is directed onto the cantilever and reflected from the cantilever to illuminate a PSD. The optical lever arrangement has proved to be a useful technology, but it has disadvantages. The user must realign the laser on the probe or realign the PSD, or both every time a probe is replaced. Furthermore, the optical lever arrangement requires a complicated apparatus consisting of a light source (typically a laser), a PSD, lenses and electronics which complicates the AFM and limits its usefulness in some applications. AFMs could be greatly improved if the probe's mechanical response could be detected by the probe itself without the use of the optical lever.